Wavelength division multiplexing (WDM) is a technique used to transmit multiple channels of data simultaneously over the same optic fiber. At a transmitter end, different data channels are modulated using light having different wavelengths or, colors if you will, for each channel. The fiber can simultaneously carry multiple channels in this manner. At a receiving end, these multiplex channels are easily separated prior to demodulation using appropriate wavelength filtering techniques.
The need to transmit greater amounts of data over a fiber has led to so-called Dense Wavelength Division Multiplexing (DWDM). DWDM involves packing additional channels into a given bandwidth space. The resultant narrower spacing between adjacent channels in DWDM systems demands precision wavelength accuracy from the transmitting laser diodes.
Unfortunately, as laser diodes age, they are known to exhibit a wavelength drift of up to 0.15 nm from their set frequency over about a fifteen year period. In a DWDM system this wavelength drift is unacceptable as a given channel may drift and interfere with adjacent channels causing cross talk. Thus, most laser transmitters use what is commonly referred to in the art as a wavelength locker to measure drift frequency vs. set frequency. This information can be fed back to a controller such that, various parameters, such as temperature or drive current, of the laser diode can be adjusted to compensate for the effects of aging and keep the diode laser operating at its set frequency. Most laser transmitters with an integrated wavelength locker use either an etalon or thin film filter to measure the laser wavelength variation. In order to work accurately, it is important that the locker be tuned by precision alignment with the collimated beam being monitored. A typical wavelength locker requires a placement accuracy of the etalon and collimated beam within better than +/− 0.5 degree.